Electric motor driven knife sharpener



Aug. 11, 1959 T. E. BEDNARSKI 2,893,709

' ELECTRIC MOTOR DRIVEN KNIFE SHARPENER Filed April 1, 1957 2Sheets-Sheet 1 5 1 ZIQIII M) III/III g- 1959 T. E. BEDNARSKI 2,898,709

ELECTRIC MOTOR-DRIVEN KNIFE SHARPENER FiledApril 1, 1957 2 Sheets-Sheet2 2,898,709 Patented Aug, 11, 1959 2,898,709 ELECTRIC MOTOR DRIVEN KNIFESHARPENER Thaddeus E. Bednarski, Milwaukee, Wis., assignor to John OsterManufacturing Co., Milwaukee, Wis., a corporation of WisconsinApplication April 1, 1957, Serial No. 649,744 4 Claims. (Cl. 51-102)This invention relates to knife Sharpeners and refers more particularlyto electric motor driven knife sharpeners of the type illustrated in theMcMaster et a1. Patout No. 2,775,075. Elecric knife sharpeners of thistype have a single abrasive Wheel, and the grinding or sharpening isdone on the opposite faces of the wheel as distinguished from theperiphery thereof, as in the sharpener of the Fillweber Patent No.2,707,852.

One of the difficulties encountered in designing a commerciallypracticable motor driven knife sharpener, as discussed at length in theaforementioned McMaster et al. Patent No. 2,775,075 resides in providingassurance against damage to the knife blade resulting from abnormalpressure of the blade on the wheel. In the Me- Master et a1. patent,this problem is met by a particular relationship between the diameter ofthe wheel, the speed of the motor and the power delivered by the motor,to the end that the motor stalls before the knife can be damaged. i

The present invention has as its purpose, a simpler solution to thisproblem of assuring against damage due to excessive pressure of theknife blade against the abrasive Wheel. The invention achieves thisobjective through a novel impositive torque transmitting connectionbetween the motor shaft and the abrasive Wheel by which the wheel'isdriven only so long as the pressure of the knife blade against the wheelis less than a safe maximum. More specifically, this torque transmittingconnection between the motor shaft and the abrasive wheel comprises twooppositely acting spring loaded friction slip-clutches, one at each sideor face of the wheel. These clutches constitute the sole means oftransmitting torque to the wheel which 'is otherwise free on the shaft,both axially and rotatably.

The clutches are so designed and constructed that together they arecapable of overcoming the braking effect of the knife blade against theabrasive wheel under normal use of the sharpener, but individually theycannot drive the wheel against such braking efiect.

Hence, in the event of abnormal pressure against either face of thewheel by the knife being sharpened, and the resulting axial displacementof the wheel, the slip-clutch at the engaged face of the wheel isdisengaged or disabled, and although the spring loading of the otherclutch is increased by such axial displacement of the wheel, the torquetransmitted thereby is incapable of overcoming the braking effect.Consequently, the wheel stops before the knife blade is damaged.

Although the sharpener of this invention is primarily intended for thesharpening of knives, it is also adapted to the sharpening of scissors.Such sharpening is done on the periphery of the abrasive wheel, and toguard against damaging scissors blades by a too rapid grinding action,the invention has as another of its objects to provide a predeterminedrelationship between the shape of the wheel periphery and the rest whichsupports and guides the scissors blades during the sharpening thereof,by which relationship abnormal pressure of the blade against the wheelperiphery results in axial displacement of the wheel and consequentdisengagement of one of the clutches.

Another object of this invention is to protect the two spring loadedfriction slip-clutches from being rendered inoperative by the abrasiveparticles resulting from the sharpening action. Specifically, thisobject of the in-, vention is achieved by enclosing the slip-clutches inpockets formed in the opposite faces of the abrasive wheel.

With the above and other objects in view which will appear as thedescription proceeds, this invention resides in the novel construction,combination and arrangement of parts substantially as hereinafterdescribed and more particularly defined by the appended claims, it beingunderstood that such changes in the precise embodiment of thehereindisclosed invention may be made as come within the scope of theclaims.

The accompanying drawings illustrate one complete example of thephysical embodiment of the invention constructed according to the bestmode so far devised for the practical application of the principlesthereof, and in which:

Figure 1 is a view partially in side elevation and partially inlongitudinal section through a knife sharpener embodying this invention;

Figure 2 is an exploded perspective view of the motor. shaft, theabrasive wheel, and the parts which together form the two slip-clutches,one at each side or face of the Wheel, the wheel being shown in crosssection;

Figure 3 is a cross sectional view through the abrasive, Wheel showingthe same properly mounted upon and drivingly connected with the motorshaft;

Figure 4 is a top plan view of the sharpener, illustrating how it isused in sharpening scissors blades; and

Figure 5 is a cross sectional view through Figure 4 on the plane of theline 5--5.

Referring now more particularly to the accompanying drawings, thenumeral 5 designates the housing or body of the entire knife sharpener,and which preferably consists of two mating upper and lower sections 6and 7. The upper housing section 6 has a transversely extendingdepression or recess 8 in its top wall defined by two downwardly slopingand forwardly converging side walls 9 and a bottom wall 10 joining theadjacent lower edges of these sloping side walls. The bottom wall 10 hasa hole 11 through which an abrasive wheel 12 projects upwardly into thespace between the walls 9. The walls 9 thus provide guide surfaces tosupport and properly position a knife blade to be sharpened byengagement with first one and then the other face of the abrasive wheel.The opposite faces of the abrasive wheel are bevelled, as shown, so thatforwardly of the center of the wheel its opposite faces are tangent toplanes substantially parallel to the sloping walls 9. The periphery ofthe wheel is frusto-conical to adapt the sharpener for use in sharpeningscissors, as will be more fully explained hereinafter.

A guard plate 13 is preferably fixed across the top of the depression 8;and, obviously, of course, the side edges of this plate must be spaced ashort distance from the sloping walls 9, as shown, to permit knifeblades to be presented to the wheel. The guard plate 13 rests upon apedestal 14 forming part of the upper housing section, and is suitablysecured to the back of the housing as at 13'.

The abrasive wheel 12 is mounted on the shaft 15 of an electric motor16, located within the housing and suitably fixed to a mount 17 formedas part of the bottom wall of the housing. The motor need not be of anyparticular'type, as long as it is adequate to drive the abrasive wheelat the desired speed.

The manner in which the abrasive wheel is mounted upon and drivinglyconnected with the motor shaft constitutes the chief feature of thisinvention. As already briefly explained, two similar spring loadedfriction slipclutches' 18, one confined between each side or face of theabrasive wheel and an adjacent abutment on the shaft, provide the soletorque transmitting connection between the shaft and the wheel, which isotherwise free of the shaft, both torsionally and axially. One of theshaft abutments is a shoulder 19 provided by reducing the diameter ofthe shaft, and the other is provided by a friction type lock nut 20threaded onto the reduced diameter shaft portion 21. The wheel is thuslocated between the two shaft abutments and, since the springs of thetwo clutches act in opposition to one another, they hold the wheelsubstantially centered with respect to the abutments. Hence, if themotor is so positioned that the shaft abutments 19 and 20 are centeredwith respect to the knife guides 9, the clutches will yieldingly holdthe wheel in proper relation to the knife guides. Since the abutments 19and 20 are directly on the motor shaft, it follows that to assure propercentering thereof with respect to the knife guides, the motor must beaccurately positioned and to assure such proper positioning, the holesin the motor mount 17 through which the motor fastening screws 16' passare elongated in a direction parallel to the axis of the motor.

Although the specific construction of the slip-clutches is subject tomodification, good results have been obtained through the use of a bowedspring Washer 22 confined between an outer washer 23 which bears againstthe adjacent shaft abutment, and a pair of contacting fiat washers 24and 25 which bear against the side of the wheel. The fiat washers 24 arepreferably thinner than the washers 23 and 25. By virtue of the tensionimposed upon the spring washers 22 by means of the nut 20, thefrictional engagement between the contacting surfaces of the severalwashers and between the endmost washers and the shaft abutments and thesides of the wheel yieldingly resists relative rotation between theshaft and the wheel.

The magnitude of these friction forces is, of course, dependent upon thesetting of the nut 20, which should be so adjusted that the tension uponthe clutch springs 22 issuch that together the two clutches are capableof transmitting sufficient torque from the shaft to the wheel toovercome the braking effect upon the wheel during normal use of thesharpener so that the wheel turns with the motor shaft. However, if aknife blade is pressed against the wheel with a force great enough tocause damage to the blade, the resulting side thrust upon the wheel willshift the wheel axially out of its normal positionand thereby decreasethe spring tension of the clutch at the engaged face of the wheel to thepoint where that clutch no longer transmits torque. In other words,under such abnormal pressure, one of the clutches is disabled ordisengaged, and although the tension upon thespring in the other clutchis increased by such axial displacement of the wheel, that clutch aloneis incapable of overcoming the braking effect of the knife upon thewheel, so that the wheel stops.

As will readily appear, since the friction clutches are duplicated atopposite sides or faces of the abrasive wheel, it makes no difference atwhich side of the wheel the excessive pressure is exerted. In eitherevent, the rotation of the wheel is stopped before damage to the knifeblade can result. Of course, stoppage of the wheel in this way does notstop the motor, which continues to run and turn the shaft.

To prevent the slip function of the clutches from being destroyed by anaccumulation therein of abrasive particles resulting from the grindingor sharpening action, the clutches are nested in counterbores 26 at theopposite ends of the hole 27 which extends Centrally through the 4 wheeland by which the wheel is loosely fitted on the shaft. Thesecounterbores are substantially closed by the washers 23 which bearagainst the shaft abutments. The washers 23 thus not only constitutepart of the slip-clutches but also cooperate with adjacent surfaces ofthe Wheel to from enclosures for the clutches.

In the sharpening of scissors blades, the edge of the blade is heldagainst the frusto-conical periphery of the wheel as indicated in Figure4. To assure proper presentation of the scissors blade to the peripheryof the wheel, the upper housing section 6 has its front shaped to exposethe abrasive wheel at the front thereof and to provide a ledge 28 whichextends across the front of the wheel and lies in a downwardly andrearwardly sloping plane which intersects the wheel and is tangent to acircle concentric to the wheel axis and slightly larger than the shaftdiameter so that a scissors blade resting flatly on the ledge is inposition to have its edge properly ground as the blade is movedlongitudinally across the periphery of the wheel.

It is important to observe that the described relationship between thefrusto-conical wheel periphery and the disposition of the ledge 28 issuch that as a blade resting on the ledge is pressed against the wheelperiphery a side thrust is imposed upon the wheel tending to shift thewheel axially toward the motor. Thus, in the event of abnormal pressureof a scissors blade against the wheel, one of the two clutches,specifically the one adjacent to the nut 20, becomes disengaged and thewheel stops before damage can be done to the scissors blade.

From the foregoing description, taken in connection with theaccompanying drawings, it will be readily apparent to those skilled inthis art, that this invention provides an extremely simple and reliableway of assuring against damage to a knife or scissors blade duringsharpening thereof by an electric motor driven abrasive wheel.

What is claimed as my invention:

1. An electric motor driven knife sharpener of the type wherein anabrasive wheel is carried directly upon the shaft of a drive motor andmeans are provided to guide the application of a knife blade to besharpened against the opposite faces of the wheel, characterized by: thefact that the abrasive wheel has a bore at its center by which the wheelis loosely mounted on the shaft, and a counterbore at each end of saidbore; the provision of opposing abutments on the motor shaft, oneadjacent to each face of the wheel; the provision of a washer bearingagainst each abutment and substantially closing the mouth of theadjacent counterbore, said abutments locating the washers in spacedrelation to the bottoms of the counterbores; and the provision of meansfor impositively transmitting torque from the shaft to the wheelcomprising spring means confined under tension between the bottom ofeach counterbore and the adjacent washer and cooperating with the latterto provide two fricton slipclutches between the motor shaft and theabrasive wheel, the spring means of the two clutches acting inopposition to one another and yieldingly holding the wheel in a positionsubstantially centered with respect to said shaft abutments and fromwhich position the wheel is axially shiftable by thrust applied againsteither face thereof, and the friction forces of said clutches being suchthat together the clutches are capable of transmitting sufficient torquefrom the shaft to the abrasive wheel to overcome the braking effect onthe wheel resulting from normal use of the sharpener, whereasindividually said clutches are incapable of overcoming said brakingeffect, so that axial shifting of the wheel resulting from abnormal sidethrust against either face thereof disables the clutch at the engagedface of the wheel and thus disrupts the torque transmitting connectionbetween the shaft and the wheel.

2. The knife sharpener of claim 1, further characterized by the factthat one of said abutments on the motor shaft is provided by a nutthreaded onto an end portion of the motor shaft and by axial adjustmentof which nut the tension of said spring means may be adjusted.

3. In an electric motor driven knife sharpener, the

combination of: an electric motor; a shaft driven by the motor; anabrasive wheel loosely fitted on the shaft; blade supporting and guidingmeans adjacent to the wheel to support and guide a blade to be sharpenedinto proper engagement with the wheel, said means being so disposed thata blade supported and guided thereby, upon being pressed against thewheel, imposes side thrust on the wheel to thereby tend to shift thewheel axially on its shaft; and common means for impositivelytransmitting torque from the shaft to the abrasive wheel and foryieldingly holding the abrasive wheel in a predetermined operativeposition with respect to the blade supporting and guiding means, saidcommon means comprising a pair of axially spaced abutments on the shaft,one adjacent to each face of the abrasive wheel, and a spring loadedfriction slip-clutch connected between each of said abutments and theadjacent face of the abrasive wheel, the springs of said clutches actingin opposition to one another and the friction forces of the clutchesbeing such that together the two clutches deliver sufiicient torque fromthe shaft to the wheel to overcome the braking eifect on the wheelresulting from normal use of the sharpener, whereas said clutches areindividually incapable of overcoming said braking effect so that axialshifting of the wheel resulting from abnormal side thrust on the wheelbrought about by excessive pressure of the blade against the wheel,disables one of the clutches and thus disrupts the torque transmittingconnection between the shaft and the wheel.

4. The electric motor driven sharpener of claim 3 further characterizedby the fact that the periphery of the abrasive wheel is frusto-conical,and by the fact that the blade supporting and guiding means comprises aledge extending crosswise of the periphery of the wheel and lying in aplane which intersects the wheel and is tangent to a circle concentricto and spaced from the wheel axis but considerably smaller than thewheel diameter so that a blade resting on the ledge and pressed againstthe periphery of the wheel imposes side thrust on the wheel.

References Cited in the file of this patent UNITED STATES PATENTS1,370,964 Hansen Mar. 8, 1921 1,445,676 Furber Feb. 20, 1923 1,535,611Le Roux Apr. 28, 1925 2,114,106 Geveke Apr. 12, 1938 2,638,720 Hock May19, 1953 V FOREIGN PATENTS 1,091,563 France Oct. 27, 1954

